1. Field of the Invention
The present invention relates to an optical system called a rear conversion lens which is mounted in the optical path between a master lens and the image plane to change the focal length of the total system. More particularly, the present invention relates to a rear conversion lens suitable for those compact cameras which have a view-through finder.
2. Description of the Prior Art
As the optical system used to increase or decrease the focal length of a photographing lens there have been known and used two different types of conversion lenses, i.e., front conversion lens and rear conversion lens. When used, the front conversion lens is mounted on the object side of the photographing lens, whereas the rear conversion lens is mounted on the image side of the photographing lens. The front conversion lens has some disadvantages. It is large in lens size. Especially, the front lens component of the front conversion lens needs a large diameter, and reduction of the size is practically unrealizable. In addition, as it is attached in front of the master lens, the front conversion lens projects from the camera body. It is difficult for a camera body to have such a front conversion lens within it as a built-in lens. Compared with the front conversion lens, the rear conversion lens has the advantages that it is possible to further reduce the size and that it is easy to contain the conversion lens within a camera body because the lens is mounted at a position near the image plane.
Conventional rear conversion lenses hitherto well-known in the art are, first of all, those which have been adapted for the objective lenses for singlelens reflex cameras. For example, they are disclosed in Japanese Patent Publication Nos. 12,421/1976; 34/813/1983; 13,015/1976 and Japanese Laid Open Patent Application No. 123,515/1983.
For the master lens in a single lens reflex camera, a longer back focus than generally determined is required and, therefore, the position of the exit pupil is far from the image plane as compared with the lens for a compact camera. FIG. 1 illustrates this. In FIG. 1, P is an exit pupil, Q is the center of the pupil and Y is the maximum image height. .theta. is the angle which the ray running toward the maximum image height (Y) forms with the optical axis. This angle .theta. is referred to as the exit angle. The lenses for a compact camera generally have a large exit angle but the lenses for a single lens reflex camera have a small exit angle. Therefore, when a rear conversion lens suitable for the master lens for a single lens reflex camera is mounted to a compact camera lens, it is no longer possible to obtain sufficiently well-corrected aberrations because of the difference in exit angle between the two camera lenses. The prior art rear conversion lens disclosed in the above-referred Japanese Patent Publication No. 13,015/1976 is relatively simple in structure. However, since the distance between two lens groups is large, the conversion lens is of large size which is unsuitable to be contained in the body of a camera. The rear conversion lenses disclosed in Japanese Patent Publication Nos. 12,421/1976 and 34,813/1983 are also unsuitable to be build in a camera body. They are composed of five or six lenses and, therefore, they are complex in construction. In addition, the total length thereof is too large to be a built-in lens. The conversion lens proposed by Japanese Laid Open Patent Application No. 123,515/1983 is a very thin tele-conversion lens. It is composed of only four of five lenses and of simple construction. However, its magnification is low which is .times.1.4324 at maximum. There are also known many other tele-conversion lenses adapted for telephoto lenses. However, they are generally unsuitable for compact cameras because the exit pupil is distant from the image plane.
Examples of the tele-conversion lenses designed for compact camera are disclosed in Japanese Patent Publication No. 32,681/1983 and Japanese Laid Open Patent Application Nos. 94,318/1981, 95,210/1981 and 46,224/1982. These tele-conversion lenses have the following drawbacks:
The conversion lens disclosed in the first mentioned patent publication No. 32,681/1983 is composed of six lenses which is too many to obtain a compact construction. So, the drawback of the known conversion lens is its complex structure.
The conversion lens disclosed in the Japanese Laid Open Patent Application No. 94,318/1981 is composed of two components, a convex lens and a concave lens. The conversion lenses proposed by the Japanese Laid Open Patent Application Nos. 95,210/1981 and 46,224/1982 are of three component type comprising convex, concave and convex lenses. These conversion lenses are all compact but involve a problem of the generation of coma. In these prior art conversion lenses, aberrations are corrected in such manner as to correct mainly the remaining aberrations of the master lens. Therefore, when the conversion lens and the master lens are coupled together, sufficiently good optical performance is not obtained. In the specifications of these prior applications there have been shown some embodiments of the conversion lens having 1.514 magnification. However, it has been found that at higher magnification, the problem of coma in the prior art conversion lenses is aggravated by the effect of higher order aberrations.